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The present invention relates to hydrostatic vehicle power steering systems, and more particularly, to such systems for use on vehicles which typically travel at relatively higher speeds, including, but not limited to xe2x80x9con-highwayxe2x80x9d vehicles, such as automobiles and light trucks, as well as xe2x80x9chigh speedxe2x80x9d tractors, i.e., those intended to travel at speeds in excess of about 30 mph. For simplicity, both types of vehicles will hereinafter be included within either of the terms xe2x80x9con-highwayxe2x80x9d and xe2x80x9chigh speedxe2x80x9d.
More specifically, the present invention relates to hydrostatic vehicle power steering systems of the type including a full fluid-linked steering control unit (SCU) which controls the flow of fluid from a pressure source (such as a power steering pump) to a fluid pressure operated actuator associated with the steered wheels. Operation of the SCU occurs in response to a manual input by the vehicle operator. The system of the type to which the present invention relates further includes some form of compensation valving which can either add fluid to the actuator or subtract fluid from the actuator (i.e., to or from the conduit connected to the inlet of the actuator), in response to sensing an xe2x80x9cerrorxe2x80x9d between the steering input (steering wheel position) and the steering output (steered wheel position). A system of this type is illustrated and described in U.S. Pat. No. 6,076,349, assigned to the assignee of the present invention and incorporated herein by reference.
Implementation of a steering system of the type shown in the above-incorporated patent has involved placing a sensor somewhere on the fluid pressure actuator, to generate a steered wheel position, and placing a sensor near the steering column, to generate a signal representative of steering wheel position.
An important aspect of the SCU illustrated and described in the above-incorporated patent was to increase substantially the stiffness (spring rate) of the recentering springs, such that flow to the steering actuator may occur, in response to the initiation of steering wheel rotation, even in the absence of relative displacement of the spool valve and sleeve valve within the SCU. However, the positional relationship between the steering wheel and the fluid meter (which is determined by the stiffness of the recentering springs) is still a relatively softer connection than the positional relationship between the fluid meter of the SCU and the steered wheels, this latter positional relationship being related primarily to the compressability of the fluid and the compliance of the various system elements, including the hoses (conduits). This relatively stiff relationship between the fluid meter and the steered wheels is, however, subject to the phenomenon of leakage, which is one of the primary reasons for the system including the capability to add xe2x80x9ccompensationxe2x80x9d fluid.
In connection with the development of the system of the above-incorporated patent, it has been determined that one or more problems occur whenever the system controller detects an error between steered wheel position and steering wheel position and begins to compensate by adding fluid between the SCU and the steering actuator. The compensation fluid added to the circuit causes a pressure rise in the steering circuit and this pressure rise impacts the fluid meter, the position of which is determined primarily by the position of the steering wheel and the relative deflection of, or displacement between the spool valve and sleeve valve of the SCU. There is a relatively softer connection (recentering springs) between the steering wheel and the fluid meter, as was described previously. One result of the pressure rise impacting the fluid meter is an undesirable increase or decrease in the resisting torque of the steering wheel, as felt by the vehicle operator.
Another problem which occurs with the system described above is the uncertainty of the relative deflection of the spool valve and sleeve valve of the SCU. This uncertainty is one of the defining limits to the controllability of the system. In other words, the difference, or error, between the steering wheel position and the steered wheel position can not be reduced below the total amount of uncertainty of the system, and the instantaneous spool-sleeve deflection may add substantially to that uncertainty.
In steering systems of the type to which the present invention relates, in which it is necessary to sense steering input motion, the performance of the steering system can be improved by increasing the resolution of the sensor which generates the command position signal. As used herein, the term xe2x80x9cresolutionxe2x80x9d refers to the number of discrete points identifiable by the controller (vehicle microprocessor) per unit of travel.
Accordingly, it is an object of the present invention to provide an improved hydrostatic steering system adapted for an on-highway type of vehicle, or for a high-speed off-highway vehicle, which overcomes the above-described problems of the prior art system.
It is another object of the present invention to provide an improved hydrostatic steering system which accomplishes the above-stated object and which substantially improves the operator feel during a steering maneuver.
It is a further object of the present invention to provide an improved steering control unit in which a command position signal having a higher resolution can be generated easily, and without the need for substantial, expensive structure being added to the steering control unit.
The above and other objects of the invention are accomplished by the provision of an improved full fluid-linked steering system adapted to provide input movement to a pair of steered wheels of a vehicle, in response to a manual input to a steering member, the steering system comprising a source of pressurized fluid, a fluid controller, and a fluid pressure operated actuator adapted to be operably associated with a pair of steered wheels to provide the input movement thereto in response to the manual input to the steering member. The fluid controller includes a housing defining a fluid inlet port in fluid communication with the source of pressurized fluid, and a control port in fluid communication with the actuator, the fluid controller further including a fluid meter having a moveable member operable to measure fluid flow through the fluid meter. The fluid controller also includes a valve means operable to control fluid flow from the fluid inlet port to the fluid meter and to the control port in response to the manual input to the steering member. A spring biases the valve means toward a neutral position. A steered wheel position sensor is operable to transmit to a vehicle microprocessor a signal representative of instantaneous steered wheel position.
The improved steering system is characterized by an input position sensor operably associated with the fluid controller and sensing movement of the moveable member of the fluid meter, to transmit to the vehicle microprocessor a signal representative of instantaneous position of the fluid meter. The vehicle microprocessor includes a comparator for comparing the signal representative of the position of the fluid meter and the signal representative of steered wheel position and generating a command signal. The system further includes a correction valve having an inlet in communication with the source of pressurized fluid and an outlet in fluid communication with the actuator, the correction valve receiving the command signal and correcting the fluid flow to the actuator to tend to null the difference between the input position signal and the steered wheel position signal.
In accordance with another aspect of the invention, the fluid controller comprises a rotary fluid pressure device comprising a housing defining a fluid inlet port and a fluid outlet port, a fluid displacement mechanism associated with the housing, and including an internally-toothed ring member and an externally-toothed star member. The star member is eccentrically disposed within the ring member for orbital and rotational movement therein. The ring member and the star member inter-engage to define a plurality N of expanding and contracting fluid volume chambers in response to the orbital and rotational movement. A rotatable valve means is operably associated with the housing and provides fluid communication between the fluid inlet port and the expanding volume chambers and between the contracting volume chambers and the fluid outlet port. A sensor assembly is operably associated with the rotary fluid pressure device to provide an output electrical signal representative of the operation of the device.
The improved rotary fluid pressure device is characterized by the sensor assembly including a first member operably associated with the star member to rotate at a speed representative of the speed of the orbital movement of the star member. The sensor assembly further includes a sensor element operable to sense the speed of rotation of the first member and generate the output electrical signal representative of displacement of the star member.